In optical fiber communication systems, communication channels can be provided by transmitting signals impressed on laser beams having different wavelengths (WDM). Although optical fiber communication systems utilizing wavelength-distinct modulated channels may carry information over long distances, signals transmitted through optical fibers are attenuated mainly by the cumulative and combined effects of absorption and Rayleigh Scattering. While the signal attenuation per kilometer in optical fibers used for communications is typically low, signals transmitted over increasing transmission distances require periodic amplification.
Stimulated Raman Scattering induces a power tilt over the signals within the C-band and L-band range of transmission frequencies. The power tilt is linear in first order on a logarithmic scale and depends on the overall signal and noise power injected into the fiber and on the fiber Raman coefficient. The tilt does not depend on the distribution of the channels within the band. For instance, the tilt per span amounts to about 0.7 dB per 100 mW signal power in single mode optical fiber (SMF) and about 1 dB per 100 mW signal power in LEAF. In this example, tilt can be defined as the difference in span loss between channel #80 (around 1603 nm) and channel #1 (around 1570 nm). The tilt accumulates linearly (in dB) with the number of spans of optical fiber.
FIG. 1 shows a simplified view of a transport system including a fiber span 210 as is typical in the art. Several inline amplifiers (ILAs) 200 are connected in series in order to transport a signal hundreds or thousands of kilometers.
In the prior art, the Raman power tilt is compensated for by introducing a gain tilt in an ILA. FIG. 2A shows the input power to into ILA 200. FIG. 2B shows the power tilt of the signal after Stimulated Raman Scattering on the fiber. FIG. 2C shows the ILA compensation gain tilt used to correct or compensate for the power tilt. FIG. 2D shows the corrected output of ILA 200.
When channels are accidentally dropped, for example, due to a fiber cut or equipment failure, ILAs 200 maintain the overall gain constant by adjusting the amplifiers to take into account the lower ILA input power. However, as a lower channel count on the fiber leads to a reduced power tilt, the ILAs typically overcompensate for the power tilt and introduce a negative power tilt as shown in FIG. 5.
FIG. 5A shows the fiber input power of a signal with fifty percent fewer channels than shown in FIG. 2A. FIG. 5B shows the power tilt over the signal after Stimulated Raman Scattering. FIG. 5C shows the ILA compensation gain tilt used to correct or compensate for the power tilt. FIG. 5D shows a gain tilt adjustment after the loss of channels. The ILAs now overcompensate for the power tilt and introduce a negative power tilt. The negative power tilt accumulates in the system and can typically reach values of more than 5 dB. For example, if 20 channels are dropped, at 3 dBm each, with 20 spans of LEAF a greater than 8 dB power tilt can be introduced. Such a power tilt usually causes system failure.
Certain prior art systems have attempted to address this problem with varying success.
U.S. Pat. No. 6,088,152 to Berger et al, entitled “Optical Amplifier Arranged To Offset Raman Gain”, discloses an invention wherein an optical amplifier adjusts the gain that the amplifier applies to optical signals so the gain favors the signal components at the low end of the bandwidth. A program uses a value representing power into the fiber to index a table of pre-tilt values. The program then determines the difference between a pre-tilt value read out of the table and a pre-tilt value obtained from an optical monitor and adjusts a variable attenuator unit as a function of the difference to obtain the desired pre-tilt of the output signal. The Berger system is comparatively slow. It also requires a static look up table that must be reprogrammed upon certain fiber or amplifier changes.
U.S. Pat. No. 6,356,384 to Islam, entitled “Broadband Amplifier And Communication System”, discloses a tilt control device to control gain tilt coupled to splitters, Raman amplifiers, EDFAs and combiners. Islam does not, disclose or suggest a method or controller system for fast automatic tilt correction of gain control.
United States Patent Application Publication No. 2002/0044317 to Gentner, et al., entitled “Control Method And Optical Data Transmission Path For Compensating Changes In Srs-Induced Power Exchange”, discloses a device for determining the tilt of all optical spectrums, and then using a quick control and slow control for compensating the tilt. Gentner requires laser equipment for quick response to power fluctuations which raises system cost and complexity.
Therefore, a need exists for a method and a device which permits compensation of the titling of the spectrum fast enough to prevent system shutdown upon loss of channels, and cheaply enough to reduce the cost of deployment in an optical transport system.